The present application is related to one commonly assigned, co-pending application having a docket numbers D 4384, filed concurrently herewith.
The present invention is directed to forming fibers having cores with non-circular cross-sections, single-mode or multi-mode, with step-index or graded-index profiles, and the fibers, fiber lasers and amplifiers formed thereby. These fibers are advantageously used in brightness converter fiber laser devices to pump single mode fibers amplifiers, such as Er-doped fiber amplifiers, using broad area multi-mode laser diode pumps.
Originally, single mode fiber lasers and amplifiers were pumped using single mode semiconductor laser diodes. However, these semiconductor pump lasers do not output very much power, limiting the brightness output by the fiber laser or amplifier. In order to increase the power output by the pump lasers, the laser diodes were designed to be broad area lasers, which output a multi-mode beam having a beam cross-section with a large aspect ratio. Additionally, the beam along one axis will be much more divergent along one axis, i.e., the xe2x80x9cfast axisxe2x80x9d than along an orthogonal axis, i.e., the xe2x80x9cslow axisxe2x80x9d. For example, in the plane of a laser diode junction, the slow axis beam divergence has a numerical aperture in the range of 0.07 to 0.15, while in the orthogonal plane, the fast axis has a higher numerical aperture in the range of 0.55 to 0.7.
The output power of such broad-area lasers is in the range of 1 to 10 W. compared to only 0.2 to 1 W from single-mode semiconductor lasers. Special fibers, such as those in accordance with the present invention, are required to collect and transform the pump power from such high power multi-mode beams into single mode beams that can be coupled into and absorbed by single mode fibers, e.g., Er fibers, that deliver amplified output power in excess of 1 W.
While providing a fiber with a core having a diameter equal to the width of the multi-mode pump beam will capture the entire beam, such coupling does not optimize brightness, which is the power per unit area per unit solid angle. Thus, the output brightness of the fiber is not significantly improved. Further, the use of such a large core does not address the problem of converting multi-mode pump light into as a single mode output.
Since the beam output by the broad area laser is a multi-mode beam, the difference in cross-section between the beam and the core can be compensated for by shaping the beam to match the shape of the core. However, any such shaped beam will still be a multi-mode beam, and attempts to couple such a shaped beam into a single-mode fiber, such as an erbium doped fiber amplifier, will lead to poor coupling efficiency. In general, if the multi-mode beam consists of ten modes, the coupling of power into a single mode core will be less than {fraction (1/10)}.
Attempts at using multi-mode laser diode beams to pump a single mode core Er amplifier involve creating a brightness converter using a solid-state laser (disclosed in commonly assigned D 14163 filed concurrently herewith), a tapered fiber laser (disclosed in commonly assigned D 14384 filed concurrently herewith) or a double-clad fiber laser. A double-clad structure includes two claddings, a first clad adjacent to a circular, single mode core, and a second clad, surrounding the first clad. The cross-section of the first clad may be designed to be a desired shape, e.g., matched to the near field emitted by the pump source or any other scheme or shape which increases absorption efficiency of the pump beam. The numerical aperture between the first and second clad layers must be large enough to capture the output of the pump laser. The actual increase in brightness realized depends on the ratio of pump cladding area to core area, with the higher the ratio, the greater the brightness. However, this disparity in area between the core and cladding cross-sections necessitates a long device length, since the absorption of the pump radiation is also proportional to this ratio.
Thus, these double-clad arrangements facilitate pumping of the fiber using a multi-mode first cladding for accepting and transferring pump energy to a core along the length of the device. Typically, a high numerical aperture, related to the difference in refractive index between the first and second cladding, is desired. Typically, the first clad layer is made of glass and the second clad layer is made of plastic, e.g., fluorinated polymer, having a relatively low refractive index, i.e., lower than that of glass, in order to increase the numerical aperture. Such plastic may not have the desired thermal stability for many applications, may delaminate from the first cladding, and may be susceptible to moisture damage. Further, the step-index double clad concept is not efficient with three-level transitions, such as the 980 nm transition of ytterbium.
The present invention is therefore directed to a method of creating fibers having cores with non-circular cross-sections, and the fibers, fiber lasers and amplifiers formed thereby, which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
Further, the present invention allows all-silica double clad fibers to be made. Additionally, the present invention enables the fabrication of new brightness converting fiber structures and geometries.
At least one of the above and other advantages may be realized by forming an optical fiber including forming a void having a non-circular cross-section in a housing, filling the void with an optical material, and collapsing and drawing the housing after said filling to form a fiber of a desired dimension.
At least one of the above and other advantages may be realized by providing an optical fiber including a housing having a void with a substantially non-circular cross-section, and a core substantially filling the void.
At least one of the above and other advantages may be realized by an optical fiber comprising a cladding and a core with an active region, the active region having a same refractive index as material immediately adjacent the active region.
At least one of the above and other advantages may be realized by providing a laser system including a multi-mode light source outputting multi-mode light, a brightness conversion fiber receiving multi-mode light from the multi-mode light source and outputting single mode light, the brightness conversion fiber comprising a cladding and a core with an active region, the active region having a same refractive index as material immediately adjacent the active region, and a single mode laser fiber receiving single mode light output from the brightness conversion fiber and outputting a laser beam.
These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.